I suspect a lot of the testing was done for 2K/4K and the requirement for systems most likely has changed. I am using an old Z800 and a 2011 MacBookPro with 8.3 but I certainly keep my performance expectations in check.
I think the 5K Mac is still in the testing phase, but if I remember correctly, had something to do with the graphics card. I reluctantly moved to Yosemite because of changes to “preview” and QuickTime, but so far I’ve been OK.
You will need to decide to get on the $299 support by end if year if you haven’t already, but it does not mean to have to install anything until ready.
Michael